Optional field trip after workshop: the Lanzo peridotite massif
(led by G.B. Piccardo)

This two-day excursion aims at discussing the field, petrologic and geochemical heterogeneities of fresh mantle peridotites that were exposed at the sea-floor of the Ligure-Piemontese basin. In particular, mantle sequences related to an ocean-continent transition setting (North Lanzo) and an internal oceanic domain of the basin (South Lanzo) will be visited.

North Lanzo - Spinel (ex-garnet) pyroxenite band in harzburgiteIn the North Lanzo body, spinel hazburgites retaining the isotopic record of long residence in the subcontinental lithosphere will be shown. The exposures comprise:

  • spinel harzburgites preserving orthopyroxene + spinel clusters that suggest garnet-bearing precursors;
  • decimetre-scale bands and veins of spinel pyroxenites displaying textural evidence of previous garnet-bearing assemblages;
  • decametre to hectometre peridotite masses enriched in plagioclase and micro-gabbroic aggregates, suggesting percolation and refertilization by MORB-type melts.

South Lanzo – Replacive dunite channelThe South Lanzo body consists of heterogeneous peridotites recording variable melt-peridotite interaction processes. Exposures that will be visited are:

  • plagioclase-enriched peridotites formed by impregnation and refertilisation by MORB-type melts;
  • replacive spinel-bearing harzburgite to dunite channels formed by focused and reactive migration of MORB-type melts.

Evolution of the Lanzo Massif during rifting of the Jurassic Ligurian Tethys. (A) The pre-Triassic setting - (B) Late Triassic-Early Jurassic stage of extension and thinning of the continental lithosphere: (i) Plg-facies exhumation of the shallower lithospheric mantle (Lanzo North protoliths); (ii) adiabatic upwelling and decompression melting of the asthenosphere; (iii) porous flow percolation of MORB-type melts through the lower mantle lithosphere (Lanzo South protoliths). - (C) Jurassic stage of migration of MORB magmas through the South Lanzo peridotites and infiltration in the North Lanzo peridotites. – (D) Late Jurassic stage of ocean opening and sea-floor exposure of North Lanzo peridotites at Ocean-Continent Transition (OCT) zones and of South Lanzo peridotites at more Internal Oceanic (MIO) settings of the basin. (1) Plg-facies lithosphere; (2) Sp-facies lithosphere; (3) Gnt-facies lithosphere; (4) Melt percolated lithosphere; (5) Melting asthenosphere; (6) Aggregate MORB formation in the asthenosphere; (7) MORB migration through the South Lanzo peridotites within replacive channels; (8) MORB melt impregnation of the Plg-facies North Lanzo lherzolites; (9) refractory residua after MORB melting; (10) extensional shear zone

The extreme compositional variability of the Lanzo peridotites records the progressive modification of pristine sub-continental lithospheric mantle by interaction with asthenospheric melts that were formed during continental extension, lithosphere stretching and asthenosphere decompression melting, related to the rifting stages of the Ligure-Piemontese basin.

The excursion does not present special walking difficulties.

Timing:

  • October 3, early morning: transportation of participants from Parma to Lanzo.
  • October 3 to October 4: field trip and overnight stay in Lanzo.
  • October 4, late afternoon: end of the field trip in Torino *

* transfer of the participants for overnight  stay in Torino (hotels in the surroundings of the  railway station area or close to the  international Torino-Caselle airport)